Resolve "(LearnerND) add iso-surface plot feature"

Closes #112 (closed)

adaptive/learner/average_learner.py

@@ -17,9 +17,9 @@ class AverageLearner(BaseLearner):
     Parameters
     ----------
     atol : float
-        Desired absolute tolerance
+        Desired absolute tolerance.
     rtol : float
-        Desired relative tolerance
+        Desired relative tolerance.
 
     Attributes
     ----------
@@ -125,3 +125,9 @@ class AverageLearner(BaseLearner):
         num_bins = int(max(5, sqrt(self.npoints)))
         vals = hv.Points(vals)
         return hv.operation.histogram(vals, num_bins=num_bins, dimension=1)
+
+    def _get_data(self):
+        return (self.data, self.npoints, self.sum_f, self.sum_f_sq)
+
+    def _set_data(self, data):
+        self.data, self.npoints, self.sum_f, self.sum_f_sq = data